1. Field of the Invention
This invention relates generally to the structure and processes of device manufacture supported on a substrate. More particularly, this invention relates to an improved substrate structure and manufacture process to provide a stable and reliable attachment of the substrate structure to a thin film supported thereon without unduly affecting the functioning and performance characteristics of the device by employing a special seam-type anchoring bonding.
2. Description of the Prior Art
A sensing device manufactured by forming a thin metallic film on a substrate often presents special technical difficulties not yet resolved by those of ordinary skill in the art Specifically, in order to achieve higher level of sensing precision and sensitivity, structural integrity of the sensing device has to be comprised. This is often caused by the fact that as the structural integrity are improved by more securely attaching the sensing element to the supporting substrate, the materials and structural elements employed often adversely interfere with the measurements designed to be performed by the sensing element thus adding imprecision or lowering the measurement sensitivity.
A specific example is a flow sensor for measuring the variations of the rate of air flow by measuring the temperature changes resulting from such variations. As disclosed by Kumada in U.S. Pat. No. 5,038,609, entitled "Thermal Type Flow Detecting Apparatus" (issued on Aug. 13, 1991), a thermal type flow detecting apparatus is patented. The flow rate detecting apparatus includes a thermoelectric detecting element, i.e., a platinum thin film, formed on a base material of a ceramic semiconductor. The apparatus further has detecting electrodes arranged in the vicinity of both longitudinal ends on one major surface of the base material where the electrodes are formed opposed to each other with the base material interposed there-between on both surfaces of the base material in a central portion between both the detecting electrodes. The heating means, i.e., the platinum thin film, is structured by a heat generating resistor fixed to the base material. As the air flows over the heating means, a temperature variation occurs which in turn changes the resistance of the resistor because of the temperature coefficient of the resistance (TCR) of the platinum film used as the sensing element. By measuring the current variation, a corresponding variation of resistance and consequently the flow rate variation can be detected.
In order to provide a highly sensitive flow rate sensor, it is desirable that the flow sensing element is as thermally insulated as possible from the supporting substrate. Therefore, a substrate of very low thermal conductivity is suitable for such applications. However, a typical thermal insulating material, e.g., a glass substrate, produces a difficulty that the sensing element, e.g., a platinum thin film cannot be securely attached to the surface of such substrate. Problems with structural integrity and reliability often becomes a concern particularly when such flow sensors are applied in environments where the flow rate is high or mechanical impact to the sensor are not preventable. Loose connection of the sensor elements from the measuring electrodes connected thereto or peeling off of the platinum thin film from the supporting substrate are often potential problems which must be taken into consideration when design a highly sensitive flow rate sensor applying thin film technology.
Many prior art attempts are applied to resolve this difficulty. Jinda et al. disclose in U.S. Pat. No. 4,805,296 entitled "Method of Manufacturing Platinum Resistance Thermometer" (issued on Feb. 21, 1989) a method of manufacturing a resistance thermometer by preparing a support substrate and forming a platinum film serving as a temperature measuring element. The platinum is formed by a sputtering process containing a predetermined amount of oxygen gas. The resistance thermometer may further include an aluminum oxide film serving as a stabilizing layer to improve the stability and reproducibility of the sensor characteristics, namely the platinum layer. The aluminum layer is particularly useful for stabilizing the resistance temperature characteristics during a high temperature treatment of the resistor thermometer. FIG. 1 is included from Jinda's patent as a background structure for understanding the technology involved in this invention. A silicon substrate 1 is provided which is covered by an aluminum oxide layer 2 for supporting a platinum film 3 thereon. The platinum film is then patterned and provided with lead wires 5 for bonding to the opposite ends of the patterned platinum film. Jinda et al. employs an inexpensive glass as substrate and applies the aluminum oxide layer to provide adhesion for the platinum film to attach to the substrate and heat resistance to sustain the heat treatment. Adding the aluminum oxide film however does not provide a complete solution to the difficulty that the platinum film may still peel off during operation of the sensor as the aluminum oxide layer still does not securely adhere to the platinum film. The structural integrity is still a problem for a high precision TCR type of device especially when thin film technology is applied to make miniaturized devices.
Similar device structures are disclosed in many other patents. Reichelt et al. disclosed in U.S. Pat. No. 4,050,052, entitled "Electrical Temperature Measuring Resistor Structure Particularly for Resistance Thermometers" (issued Sep. 20, 1977) by forming the platinum film on a layer which has a temperature coefficient of expansion matching the platinum layer. The expansion layer disclosed is an aluminum oxide layer. Frank et al. disclosed in U.S. Pat. No. 4,129,848, entitled "Platinum Film Resistor Device" (issued Dec. 12, 1978) by sputtering the platinum film on a quartz and etch the film and the quartz to define the conducting paths of the resistor device. Furubayashi et al. disclosed in U.S. Pat. No. 4,649,365, entitled "Platinum Resistor for the Measurement of Temperature" (issued Mar. 10, 1987) by forming the platinum film on a silicon substrate overlaid with an aluminum oxide film. Sittler et al. disclosed in U.S. Pat. No. 4,791,398, entitled "The Film Platinum Resistance Thermometer with High Temperature Diffusion Barrier" (issued Dec. 13, 1988) by forming the platinum film on a silicon substrate overlaid with an barrier layer which is preferably a titanium oxide layer. All these devices, with similar structure as that of Jinda et al., are still limited by the same difficulty that the device structure is subject to weak attachment of the platinum film to the substrate structure.
Therefore, a need still exists in the art of design and manufacture of thin film TCR type of devices for measuring the temperature, flow rate or other types of operation conditions, to provide a novel and improved structure to resolve the difficulties generated by this weak structure configuration. This improved device is specially required for high precision, high speed flow rate measurements where the sensing elements are constantly subject to direct blow by the air flow. Assurance of high degree of structural integrity are in therefore in great demand. It is also desirable that the structure integrity can be improved without sacrificing the measurement sensitivity whereby implementation of the TCR types of sensing device can be broadly incorporated in various modem high precision, high speed applications.